Such position measuring systems can be used, for example, in electropneumatic position controllers and electrical variable-speed drives to return the actual value and can thus be part of a closed control loop. The sudden failure of the position measuring system can result in the immediate failure of the device functionality.
The structure and method of operation of a closed control loop are known and are described, for example, at http://de.wikipedia.org/wiki/Regelkreis, extracts of which are shown in FIG. 1.
The product brochure “Der kompakte, intelligente Stellungsregler” (The compact, intelligent position controller) (ABB Automation Products GmbH, print number: 50/18-19 DE RevA; June 2005 edition, discloses an electronic position controller of the type mentioned at the outset for a pneumatic actuator.
In such a position controller and with reference to FIG. 1, the reference variable w can be preset via a desired value channel formed, in particular, by an analog 4.20 mA input or a field bus such as HART, Profibus PA, Foundation field bus, etc.
In this arrangement, the control path FS forms the pneumatic actuator/variable-speed drive to be positioned. In the known position controller, the control element FR, the actuating element FSt and the measuring element FM can be constructed in a housing. In the known actuator, the control element FR can be in the form of a microcontroller-supported system. In the known actuator, the measuring element FM can be in the form of a potentiometer with a slider tap which measures the set position x of the drive to be controlled. The actuating element FSt can typically be in the form of an IP module in an electropneumatic position controller.
In this case, the potentiometer can be supplied with a constant and known reference voltage, and the position can then be detected in an analog/digital converter using the displacement-proportional voltage tap. In terms of circuitry, this arrangement can thus be the form of a voltage divider with a position-dependent voltage tap. The feedback variable r can be present in digital form in the analog/digital converter. The voltage tap can be effected with the highest possible impedance of the measuring circuit in order to minimize measurement errors.
The microcontroller-supported system forms a controller output variable yr from the control difference e with the aid of a suitable control algorithm in the controller FR, which output variable can be used to drive the IP module via a suitable electronic circuit.
In some special applications, the position sensor belonging to the position measuring system as well as other associated components are not arranged in the same housing. In this case, the position measuring system can be arranged outside the positioner as a remote displacement sensor.
Potentiometers with a slider tap have the property that they are resistant to vibrations only to a limited extent. In addition, the slider and the resistance track can wear away as a result of electrical erosion after a finite number of movements until they are defective.
In the case of a frequently occurring error pattern, the resistance track can be damaged by abrasion and/or electrical erosion as a result of a slider which cyclically oscillates around a constantly recurring point because the position controller corrects only small control errors. This occurs, in particular, in feedback systems, as can be found in electropneumatic position controllers or electrical variable-speed drives, when they operate with a constant or virtually constant desired value for a long period of time. A defect can be fostered by poor controllability of the control path because the latter tends toward the oscillation, the period of time for which control is effected at a constant or virtually constant desired value, because the associated sensor/potentiometer range is then used for a long time, and the occurrence of a defect can increase as the frequency increases.
Only a range of a few angular degrees is often affected. In this case, the slider can work its way ever further into the material of the resistance track until it finally can no longer make contact. The potentiometer is worn at this point and therefore cannot continue to be used for measurement. More than one point of the sensor may be defective inside an operating range.
In addition, chemical influences have a negative effect on the service line of slider potentiometers. A defect of the potentiometer can result in the failure of the displacement/position measurement.
The failure of the displacement/position measuring system can result in the failure of the device function, as a result of which the position controller carries out a positioning reaction which is predetermined for the controller and in which the controller remains until the cause of the failure has been rectified. Positioning reactions which can be carried out without a displacement/position measuring system can be preset for this purpose. Depending on the respective application, provision may be made to ventilate or vent the drive, so-called “fail safe”, or to block the drive in the current position, so called “fail freeze.”
Because failure may not be predicted and is also not diagnosed at regular service intervals, such failure can result in unplanned stopping of the process which is often associated with high costs for the user.
In the attempt to increase the availability of the sensor, contactless measuring methods, as are disclosed in DE 42 39 635 A1 and DE 10 2007 019 045 A1 for example, have been investigated. However, it has been found that these methods, with a higher degree of technical complexity than potentiometer-based measurement, also have such a high energy consumption that they are rendered unusable for applications in devices which are supplied from a current loop whose power is limited.